1. Field of the Invention
The present invention relates to an image pickup apparatus whose image-taking lens can be exchanged.
2. Description of the Related Art
For single lens reflex digital cameras whose lens can be exchanged, as in conventional single lens reflex cameras for silver halide film, a focus detection unit employing so-called phase difference detection method is commonly used. Furthermore, hybrid method have also been proposed, in which focusing control of the image-taking lens is performed by a combination of a focus detection unit employing so-called contrast detection method, as used in video cameras, and a focus detection unit employing phase difference detection method.
In phase difference detection method, a portion of a light flux from the image-taking lens is divided in two, and these two light fluxes are imaged onto respective line sensors. The moving direction and the moving amount of a focusing lens that are necessary for focusing on an intended focus plane (plane conjugated to an image pickup plane) is then calculated by detecting the displacement direction and the displacement amount of the two images on the line sensors. With such phase difference detection method, the moving direction and the moving amount of the focusing lens that are necessary for focusing can be calculated directly, so that focusing can be achieved promptly.
In contrast detection method, high-frequency components are extracted from an image signal generated based on the signal that is output from the image pickup device picking up an object image, the level of these high-frequency components is observed at predetermined sampling intervals, and the focusing lens is moved in a direction in which the level of the high-frequency components approaches a peak level, and the camera is judged to be in focus when the level of the high-frequency components has ultimately reached a predetermined peak range. With this contrast detection method, the in-focus judgment is made by using an image signal obtained based on the signal output from the image pickup device picking up the object image, so that focusing with respect to the object image can be performed at high precision.
However, in single lens reflex digital cameras whose lens can be exchanged, there is the possibility that sufficient focusing precision cannot be attained when using only a focus detection unit employing the above-described phase difference detection method. The principal reason for this is that the light flux forming the object image that is observed or taken is generally different from the light flux that is used by the focus detection unit.
Moreover, in focus detection units employing phase difference detection method, the focus position or the focus displacement, which is to be decided by the amount of aberration in longitudinal (optical axis) direction, is determined by converting it into a displacement of the image, which is associated with the aberration in lateral direction, so that if there is aberration in the image-taking optical system, then it is conceivable that there is a difference between the two, depending on the state of the aberration correction.
In order to solve this, a correction circuit for correcting the focus detection signal D representing for example the focus displacement amount with a correction factor C that is unique to each image-taking lens:DC=D−C  (1)
All or a portion of the image-taking optical system is driven on the basis of the resulting corrected focus detection signal DC, and the lens is controlled such that the image pickup plane coincides with the best image-forming position.
However, in this method, a design correction factor is ordinarily used for this correction factor C, so that it does not include errors occurring at the manufacturing stage of the camera. Therefore, if errors occur at the manufacturing stage, they may become a cause of the problem that a sufficient focusing precision is not attained.
On the other hand, when using hybrid method, in which focusing control is performed by a combination of phase difference detection method and contrast detection method, in a single lens reflex digital camera whose lens can be exchanged, then a rough adjustment is carried out first by phase difference detection method, and fine adjustment is carried out by contrast detection method, thereby enabling a focusing that is to a certain degree fast and precise. However, with this hybrid method, the control up to the ultimate focusing is performed by contrast detection method, so that the time needed for the focusing cannot be shortened sufficiently.
Furthermore, in single lens reflex digital cameras having an optical finder, the destination of the light flux taken in from the image-taking optical system is switched chronologically to guide the light flux from the image-taking optical system to the finder optical system, the focus detection unit employing phase difference detection method and the focus detection unit employing contrast detection method, without compromising the light amount for those three destinations. Thus, the detection time may become long.